1. FIELD OF THE INVENTION
The present invention relates to a four-roller type rolling mill apparatus for sizing bar and rod materials to give them round sectional shapes. More particularly, the present invention relates to a four-roller type sizing mill which achieves an increased maximum rolling draft for sizing while retaining a high sizing precision.
2. DESCRIPTION OF THE RELATED ART
In general, a heated steel material is continuously rolled into a round rod having a desired size by a line of a plurality of rolling mills including roughing mills and finishing mills having grooved caliber rollers. Sizing mills are usually used at the final stages in the finishing mills.
The rolling methods using sizing mills fall into three types on the basis of the number of rollers in the first stand of the sizing mills. These types are the two-roller method, three-roller method and four-roller method, as shown in FIGS. 10(a), 10(b) and 10(c), respectively. A rolling material w is rolled through passes formed by the grooves 20a, 22a and 25a of the rollers belonging to the respective roller units which are set with respect to different rolling directions, for example, through a pass between rollers 20 and 21 and then a pass between rollers 20' and 21' in FIG. 10(a). As shown in the figures, rollers 20 to 28 have grooves 20a, 22a and 25a having predetermined sectional shapes.
Japanese Patent Application Laid-Open No. 1-202302 discloses a two-roller sizing method in which a material is rolled through three passes each of which is formed by two rollers and has rolling directions different from those of the other two passes. Japanese Patent Application Laid-Open No. 63-43072 discloses a three-roller sizing method in which a material is rolled through three passes each of which is formed by three rollers and has rolling directions different from those of the other two passes. Japanese Patent Application Laid-Open No. 62-199206 discloses a four-roller sizing method in which a material is rolled through two passes each of which is formed by four rollers and has rolling directions different from those of the other pass.
Three rolling methods as shown in FIGS. 10(a) to 10(c) were tested to compare their performances. In the test, the same type of rolling material w having a diameter d of 50 mm was sized by each of the three methods. The shapes of the grooves 20a, 22a and 25a of the rollers used to form the final-stage passes were as shown in FIGS. 11(a), 11(b) and 11(c), respectively. Radii R1, R2 and R3 of the grooves were all 25 mm, and the central angles of the grooves were: .theta..sub.1 =90.degree., .theta..sub.2 =60.degree., and .theta..sub.3 =45.degree.
The sectional views of the obtained round steel rods are shown in FIGS. 12(a), 12(b) and 12(c). The two-roller method produced a sectional shape of a quadrangular circle as shown in FIG. 12(a). The three-roller method produced a sectional shape of a hexagonal circle as shown in FIG. 12(b). The four-roller method produced a sectional shape of an octagonal circle as shown in FIG. 12(c).
The difference between the maximum diameter d.sub.1 and the minimum diameter d.sub.2 of each round steel rod (referred to as "diameter difference") was examined in relation to the rolling draft.
If a rolling draft is too large, the rolling material over-fills into the gaps between the rollers, resulting in a defect. Referring to FIG. 13, point t where the straight line segments (actually the flat areas), for example, 25c and 26c in the figure, of the grooves of two neighboring rollers, for example, 25 and 26, were defined as the points of over-filling limit, that is, the points of critical rolling draft. Rolling drafts were measured which caused the rolling material to fill up to the points of over-filling limit.
The graph in FIG. 14 is based upon these results, indicating the relation between diameter differences and rolling draft as well as the over-filling limits by arrows .dwnarw.. As indicated by the graph, the four-roller method achieves a higher sizing-precision than the other methods with respect to the same rolling draft, but the four-roller method results in over-filling with less rolling draft than the other two methods. In other words, the four-roller method can perform sizing only in limited ranges of rolling drafts. On the other hand, the two-roller method fails to achieve a high sizing-precision, but can perform sizing in a wider range of rolling drafts than the other two methods, without suffering over-filling.
FIG. 15 indicates the relation between the rolling reduction and the width expansion of round steel rods obtained by the three methods, the width expansion being obtained on the basis of the following calculation: [(the width of material after rolling--the width of material before rolling)/the width of material before rolling].times.100. As indicated by the graph in FIG. 15, the two-roller method causes a greater width expansion than the other two methods, and the three-roller method causes a greater width expansion than the four-roller method, with respect to the same rolling reduction.
If there is a large width expansion, changes in the width greatly vary depending on the type of steel material and the rolling conditions, such as temperature or rolling speed, even when the rolling draft is maintained at a constant level. In other words, the sizing-precision in the two-roller method is subject to greater deterioration. Thus, the four-roller method has, again, advantages with regard to sizing precision.
Considering the results of the experiments described above, if the sizing precision is not critical, the two-roller method is more useful than the other two methods because the two-roller method allows a greater range of rolling drafts for sizing. If the sizing precision is critical, the four-roller method is more advantageous than the other two methods.
However, since a unit of rollers of the four-roller type mills can perform sizing only in a limited range of rolling drafts, the rollers must be changed frequently in order to provide a desired rolling draft. To change rollers, the rolling operation must be stopped, thus significantly deteriorating operating efficiency.
The structure of a rolling mill becomes more complicated in the order of the two-roller method, the three-roller method and the four-roller method. More specifically, as the number of roller driving devices increases, adjustment of the roller gaps and adjustment of position of the rollers along their own axes become more delicate.
A known four-roller type mill must have four roller-driving devices because each of the four rollers needs a torque to rotate. Further, these four driving devices of the four-roller type mill must be electrically synchronized. Thus, a known four-roller type mill requires a great number of parts and components and, therefore, a complicated structure, inevitably becoming bulky and expensive.
Further, to change rollers in a known four-roller type mill, the roller driving device including motors and spindles must be removed from the main body. Still further, because the roller driving device including motors are provided around the rollers, changing or maintaining the rollers is not very easy.
As described above, though the four-roller method has an advantage in improving sizing-precision, it has several problems, such as the requirement for the complicated structure or frequently changing rollers. Therefore, the four-roller method has not been widely used in sizing mills.